CSS2.1, CSS3: background-attachment

| If a background image is specified, this property
| specifies whether it is fixed with regard to the
| viewport ('fixed') or scrolls along with the document
| ('scroll').
|
| [Note that there is only one viewport per view. If an
| element has a scrolling mechanism (see 'overflow'), a
| 'fixed' background doesn't move with it.]

There's a bit of uncertainty as to how 'background-attachment'
deals with elements that scroll.

'scroll' -
    Is the background fixed to the main canvas or to
    the element's canvas?

      "scrolls along with the document ('scroll')"

  'fixed' -
    Is the background fixed to the main viewport or
    the element's viewport?

     "a 'fixed' background doesn't move with [the element]",
     where the element could be construed as moving wrt the
     main viewport or its own viewport

Brought up from
http://bugzilla.mozilla.org/show_bug.cgi?id=165149

~fantasai